The Class D51 (D51形) is a type of 2-8-2 steam locomotive built by the Japanese Government Railways (JGR), the Japanese National Railways (JNR), and Kawasaki Heavy Industries Rolling Stock Company, Kisha Seizo, Hitachi, Nippon Sharyo, Mitsubishi, and Mitsubishi Heavy Industries from 1936 to 1945 and 1950 to 1951. 174 units are in preservation in Japan, including five operational examples. A further 13 are preserved in Russia and Taiwan, bringing the total number of preserved units to 187.

Under the Whyte notation for the classification of steam locomotives, 2-8-2 represents the wheel arrangement of two leading wheels on one axle, usually in a leading truck, eight powered and coupled driving wheels on four axles and two trailing wheels on one axle, usually in a trailing truck. This configuration of steam locomotive is most often referred to as a Mikado, frequently shortened to Mike.